1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus and an optical recording and reproducing method, particularly to an optical recording and reproducing apparatus and an optical recording and reproducing method for recording and reproducing information by using two group object lenses.
2. Description of the Related Art
In a recording and reproducing apparatus of an optical record medium, a spot size d on the record medium is represented by the following equation (1) when a wavelength of a light source is designated by notation .lambda. and an aperture number of an object lens is designated by NA (Numerical Aperture). EQU d=.lambda./NA (1)
As is known from Equation (1), the shorter the wavelength .lambda. of a light source and the larger the numerical aperture NA of the object lens, the smaller the spot size d and high density recording is made feasible.
In such a relationship, as a method of increasing the numeral aperture of an object lens, it is known to be effective to use aspherical two group object lenses. JP-A-9-251645 discloses a record medium recording and reproducing apparatus and a record medium recording and reproducing method by an applicant the same as the applicant of this application in which spherical aberration is restrained from occurring by using two group object lenses. When such two group object lenses are used, it becomes necessary to achieve optimization of a distance between the two group object lenses and minimize wave front aberration in order to make best a reproducing signal from a record medium.
In the meantime, a focal depth fd of an object lens specified by mean square error (.lambda./14) of wave front aberration, is calculated by the following equation (2). EQU fd=.lambda./NA.sup.2 (2)
As is known from Equation (2), when an object lens having a high numerical aperture is used, the focal depth fd becomes extremely small. For example, when a numerical value of two group object lenses is set to 0.85, the focal depth fd is reduced to about a half of that in the case of DVD-RAM (object lens numeral aperture NA=0.6). Here, DVD indicates a digital video disk and DVD-RAM indicates a rewritable digital video disk.
Therefore, when an object lens having a high numerical aperture is used, further accurate focusing control is requested and the lens need to follow precisely a change in environmental temperature and an aging change.
As a method of achieving optimization of a distance between two group object lenses and minimizing wave front aberration, Japanese Patent Application No. 8-340903 discloses an optical disk recording and reproducing apparatus and its method by an applicant the same as the applicant of the application in which after integrating two group lenses and carrying out focusing operation of focus servo, a front lens is independently moved in an optical axis direction by which wave front aberration is adjusted to minimize.
Further, as accurate focusing control, Japanese Patent Application No. 9-84090 discloses a focusing control apparatus and its method and an optical disk apparatus by an applicant the same as the applicant of this application in which in focus servo in an optical disk apparatus using an optical head constituted by two group object lenses having a high numerical aperture, by optimizing focus offset, a disk reproducing signal is adjusted to be best.
However, the above-described respective optimization of the distance between lenses of the two group object lenses and optimization of focus offset are carried out based on the same reproducing signal from a record medium and both are separately adjusted. Therefore, a long period of time is required in the adjustment in comparison with a case of an object lens constituted by a single piece of a lens.